1. Field of the Invention
The invention relates to a loudspeaking telephone instrument comprising a transmit path including a microphone and a receive path including a loudspeaker, the transmit or receive path further including a frequency shifter capable of shifting the frequency of an electrical signal applied to its input by a given amount.
2. Description of the Related Art
The purpose of a loudspeaking telephone instrument is to allow hands free communication, i.e. to dispense with the telephone handset by substituting for it a microphone and loudspeaker, normally mounted within the instrument casing. Thus the user can communicate without being in physical contact with the telephone instrument giving the advantage of greater flexibility, for example in referring to documents. The main problem which faces the designer of loudspeaking telephones is to overcome acoustic instability which arises from the removal of the isolation, which is inherent in the telephone handset, between the send and receive transducers.
There are two general approaches to overcoming the problem of acoustic feedback. The first is to switch off the channel which is not in control; thus breaking the feedback loop. However, the switching tends to be intrusive, a common fault being clipping or loss of syllables on switching. The second is to use adaptive echo cancellation. Such systems tend to be very complex and have stability problems associated with the degree to which the varying unwanted coupling signals can be modelled and cancelled.
A further suggestion which has been made is to include a frequency shifter in the loop between the microphone and loudspeaker as is known for public address systems and is said to enable an increase in usable gain of up to 10 dB to be obtained.